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Re: [xen-devel] System time monotonicity

To: Dave Winchell <dwinchell@xxxxxxxxxxxxxxx>
Subject: Re: [xen-devel] System time monotonicity
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Fri, 11 Apr 2008 22:41:43 +0100
Cc: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, "dan.magenheimer@xxxxxxxxxx" <dan.magenheimer@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>
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On 11/4/08 22:20, "Keir Fraser" <keir.fraser@xxxxxxxxxxxxx> wrote:

>> Its a fairly simple matter to base the hpet on the physical hpet. Its
>> easy to share it among guests
>> as no one really writes the physical hpet.  Offsets are kept in each
>> vhpet such that each guest thinks
>> he owns the hpet.
> This is really no better than basing on Xen system time. Actually it's worse
> since most systems don't even expose the HPET, so we can't probe it (without
> hacks) and so we can't use it. Xen's system time abstraction, perhaps with the
> max(last, curr) addition, is perfectly good enough.

Just to labour the point some more: If you still believe that diving to the
real platform timer on every guest time access is measurably more accurate,
you can cleanly prove that by building on Xen's system time abstraction, and
then switch between using get_s_time() (aka NOW()) and
read_platform_stime(). The latter calculates current system time by reading
from the platform timer *right now*. It's the function that all local CPUs
calibrate to once per second.

 -- Keir

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